The present invention relates to control of compensating impedance connected to supply leading or lagging reactive current to an a-c electric power system for stabilization of voltage at a critical bus area, and particularly to a control system responsive to rapid current disturbances.
It is known that electric power systems which supply highly erratic reactive loads, e.g., electric arc furnaces, are typically characterized by poor voltage regulation. Consequently, these systems often exhibit an undesirable flicker. One type of regulating system suggested to alleviate this condition is disclosed in U.S. Pat. Nos. 3,936,727 and 3,968,432, respectively issued to Kelley, Jr., et al and Kelley, Jr. on Feb. 3, 1976 and July 6, 1976. These patents are assigned to the assignee of the present application and are hereby incorporated by reference in the present application.
Generally, the above-mentioned regulating system includes the use of two control loops: one open compensating control loop; and one closed regulating or supervisory control loop. The compensating control loop senses the reactive load current component and attempts to negate the same by providing the appropriate compensating reactive current component through a reactive converter which includes capacitive and inductive components. The regulating or supervisory control loop employs a current angle sensor to sense the power factor or phase angle at a selected line location which is typically located in a critical area of the line at which good voltage regulation is desired. These two loops cooperate to generally provide a satisfactory degree of voltage regulation when an erratic load is present
One problem with such a regulating system arises because the reactive converter in the system typically includes thyristor switching for controlling the current which passes through the inductive components. The thyristor switching involves triggering the thyristor switches into conduction at an appropriate gating angle (.alpha.), i.e., the phase angle with respect to the impressed alternating voltage wave at which each thyristor is triggered into conduction. The interval during which the thyristor subsequently conducts following each triggering is referred to as the conduction angle (.sigma.). When the conduction angle (.sigma.) is substantially 180.degree. for each thyristor, the switch is considered to be fully "on" or closed; when the conduction angle (.sigma.) is substantially 0.degree., the switch is considered to be fully "off" or open. At intermediate conduction angles (.sigma.), and correspondingly intermediate gating angles (.alpha.), the switch is partially "on" and partially "off" during each half cycle and controls the amount of current flowing therethrough by the ratio of "on" time to "off" time. Once the current is initiated by gating the thyristors, current will flow for the whole half cycle if conduction is initiated at the peak of the voltage wave. Current can be changed every half cycle by control of the gating angle. Current conduction stops at every current zero and is reinitiated at the current level required by reapplication of the gate signal.
The correction provided to the power system by a change in thyristor switching may be viewed as a two-part correction. For example, advancing the gating angle (.alpha.) by .DELTA..alpha. degrees initiates conduction by .DELTA..alpha. degrees earlier in the waveform cycle and also delays the end of conduction by .DELTA..alpha. degrees, thus effecting a change in the conduction angle (.sigma.) of two times .DELTA..alpha. degrees. The geometric centroid of the conduction angle change is delayed from the gating event by half the conduction angle. This conduction angle change is determined by the control information at the instant of thyristor gating. In this connection, if the thyristor switch output is modulated by the gating control at a frequency somewhat less than power system frequency, the modulation of the output will lag the gating control modulation by approximately half the conduction angle. This lag comprises a frequency plane negative phase shift which reduces the effectiveness of the reactive converter in correcting the effects of load disturbances on the power system.
Accordingly, a general object of this invention is to provide an improved regulating system for use in an a-c electric power system wherein the regulating system includes a thyristor switched static converter.
Another object of this invention is to provide such a regulating system wherein the effectiveness of the thyristor switched reactive converter is improved by providing means for correcting for the negative phase shift produced in the thyristor switch.
Another object of this invention is to provide such a regulating system wherein the means for correcting for the negative phase shift includes positive phase shift means.